Formal is a word used to describe anything that has form or structure. So it is something which is used or done in accordance with rules or ceremonies.

Formal events have rules. 'Formal clothes' or 'formal attire' is a way of dressing to meet the rules for an event, such as a party or wedding. 'Formal attire' can be a suit, a tuxedo or a dress. People from all over the world have their own ideas about 'formal attire'. Most of the time formal attire clothes are the most beautiful or the best clothes a person owns. The opposite of "formal" is informal. Informal clothes are comfortable clothes for general use, e.g. T shirt and jeans. An informal ceremony might be a ceremony which is very relaxed instead of following an exact plan. An informal way to greet a friend might be: "Hi, John!"